The present invention relates to fan drive assemblies of the type including a cooling fan and a fan drive, and more particularly, to such fan drive assemblies wherein the fan drive is of the type in which heat is generated as a result of the transmission of torque within the fan drive, and the ability of the fan drive to dissipate such generated heat represents a limiting factor on the torque transmitting capability of the fan drive assembly.
Although the present invention may be used with various types and configurations of torque transmitting fan drives, it is especially adapted for use with fan drive assemblies of the type including a viscous fluid coupling device as the fan drive, and will be described in connection therewith.
Fan drive assemblies of the type which may benefit from the use of the present invention have found several uses, one of the most common of which is in connection with cooling the radiator of a vehicle engine. As is well known to those skilled in the art, the fan drive of the typical fan drive assembly comprises a viscous fluid coupling device, so named because the coupling utilizes a high viscosity fluid to transmit torque, by means of viscous shear drag, from an input coupling member (clutch) to an output coupling member (housing), with the cooling fan being bolted, or otherwise suitably attached, to the output coupling member.
The present invention is especially advantageous when used on a relatively high horsepower fan drive assembly, i.e., one which is capable of transmitting somewhere in the range of about two to about twelve horsepower from the fan drive to the cooling fan. Typically, such high horsepower fan drives include an output coupling member of the type which comprises a cast aluminum body and a cast aluminum cover. The input coupling member is typically also made as a cast aluminum member, and cooperates with the body and/or the cover to define a plurality of interdigitated lands and grooves which define the viscous shear space. When the shear space is filled with viscous fluid, typically a silicon fluid, torque is transmitted from the input coupling member to the output coupling assembly, in response to the rotation of the input coupling member.
During such torque transmission, substantial heat is generated as a result of the shearing of the viscous fluid between the lands and grooves. The amount of heat generated is generally proportional to the "slip speed" of the fan drive, i.e., the difference between the speed of the input and the speed of the output. It is generally well understood by those skilled in the art that the ability to transmit torque is limited by the ability of the device to dissipate the heat generated. For example, in a viscous fan drive, if the temperature of the viscous fluid exceeds a certain maximum temperature, the result will be a deterioration of the viscous properties of the fluid, resulting in a gradual loss of the torque transmitting capability of the fluid.
In the fan drive art, it has been conventional for the design and development of a particular cooling fan to occur generally independently of the design and development of the viscous fan drive with which the fan is to be utilized. In other words, the fan is designed to provide the desired operating parameters (e.g., torque, air flow, etc.), thus determining the blade configuration and spacing, and then the mounting portion of the fan (the "spider") is designed or merely modified to adapt to the configuration of the particular fan drive mounting arrangement (e.g., mounting pads or bosses, disposed at a particular diameter from the axis of the fan drive).
What has not been conventional in the fan drive art is to design the cooling fan and the fan drive as a "package", with the goal of maximizing the heat dissipation of the overall fan drive assembly. As a result, it would appear that, at the time of the present invention, there is no commercially available fan drive assembly which achieves nearly its optimum, potential heat dissipation (heat rejection). As a further result, practically every fan drive assembly in commercial use is larger and more expensive than is actually necessary, in order to achieve a particular, desired flow of cooling air through the radiator.
Although the present invention is not limited to a fan drive assembly in which the fan is mounted to the rearward side of the housing (body), rather than being mounted to the cover, the invention is especially advantageous in such an arrangement, and will be described in connection therewith. A typical "rear mount" fan drive is illustrated and described in U.S. Pat. No. 4,384,824, in which the body member includes four mounting bosses located radially inward of the body cooling fins. As a result, the fan spider interferes with the radial flow of cooling air through the body cooling fins, thus reducing the heat dissipation capability of the particular fan drive assembly.